Many available power protection devices, such as surge protection devices (SPDs) and hybrid filters, include an automatic switch circuit that disconnects power to the device output in response to various supply wiring fault and/or overvoltage conditions. As shown in FIG. 1, a conventional automatic switch circuit typically includes one or more electromagnetic relays (EMR) 10, a relay control circuit 12, an overvoltage circuit 14, and/or a wiring fault circuit 16 arranged in parallel between the supply line and neutral conductors.
Supply wiring faults, such as loss of AC ground continuity, can adversely affect the operation of both power protection devices and connected electronic equipment. Line voltage conditions, such as swells and overvoltages, can cause failure of certain power protection device components such as metal oxide varistors and other suppressor components. In addition, some equipment power supplies can be damaged by continuous overvoltage conditions.
In conventional automatic switch circuits that include a wiring fault circuit 16, continuity and correct polarity of the supply line, neutral, and ground conductors are typically required before the wiring fault circuit 16 permits the relay control circuit 12 to energize the relay (i.e., generate the necessary relay coil voltage to close the normally-open relay contacts). The relay contacts are typically configured to make/break continuity of the device line and/or neutral conductor. FIG. 1, for example, shows a relay 10 configured to make/break line continuity. Energizing the relay 10, therefore, makes continuity of the line conductor, thereby passing AC power to the device output and connected equipment. With a supply wiring fault condition (i.e., loss of continuity of any of the supply conductors and/or a reverse polarity at the branch outlet), the wiring fault circuit 16 will signal the relay control circuit 12 to remove relay coil voltage, causing the normally-open relay contacts to open, thereby disconnecting AC power to downstream device components and connected equipment. Designed correctly, automatic switch circuits that include a wiring fault circuit ensure that the branch outlet to which the device is connected is wired correctly before power is passed to the device output and connected equipment. Because wiring faults can affect operation of both the power protection device and connected equipment, ensuring a properly wired supply can be critical to the reliable operation of the electronic system (power protection device and connected equipment).
In addition to the wiring fault circuit 16, many prior art automatic switch circuits include an overvoltage circuit 14 that requires nominal line voltage levels before AC power is passed to the device output and connected equipment. In response to a line voltage condition, such a swell or overvoltage, the overvoltage circuit 14 will signal the relay control circuit 12 to remove relay coil voltage, causing the normally-open relay contacts of relay 10 to open, thereby disconnecting AC power to downstream components and connected equipment. Once the swell or overvoltage condition has subsided, the overvoltage circuit 14 will signal the relay control circuit 12 to energize the relay 10, causing the normally-open contacts to close, thereby passing AC power to downstream device components and connected equipment. Designed correctly, automatic switch circuits that include an overvoltage circuit can protect vulnerable downstream device components (e.g., metal oxide varistors (MOVs) and other suppressor components) from swell and overvoltage related damage. In addition to protecting vulnerable device components, these circuits can also protect connected equipment that might otherwise be damaged by the swell or overvoltage condition.
There are, however, limitations in conventional automatic switch circuits related to the sole use of electromagnetic relays (EMRs) for AC switching and the associated relay control circuits. Electromagnetic relays are mechanical devices that include a multi-turn coil wound on an iron core (electro-magnet), an armature, a spring, and one or more sets of contacts. Voltage across the multi-turn coil creates current flow in the coil that causes the iron core to become magnetized, thereby attracting the pivoting armature and closing the normally-open contacts. As coil voltage drops, so does current flow and core magnetic force. When the coil current drops to a level where the attractive force of the core is less than the resistive force of the spring, the armature will swing open and the relay contacts will return to their normally-open position.
Because of the mechanical nature of electromagnetic relays, there is a significant delay (turn-on response time) between application of sufficient relay coil voltage and closure of the normally-open contacts. Similarly, there is a significant delay (turn-off response time) between removal of relay coil voltage and opening of the relay contacts. These turn-on and turn-off relay response times are significantly affected by coil voltage, coil temperature, and age and history of the relay. Specifications from one relay supplier, for instance, show a turn-on response time of 8 milliseconds at an applied coil voltage of 80% of the rated coil voltage and a turn-on response time of 2 milliseconds at an applied coil voltage of 150% of the rated coil voltage. The above response time/coil voltage relationship along with variations in relay coil resistance with coil temperature and changes in the relay spring characteristics over time mean significant variations in relay turn-on and turn-off response times.
Because of this response time variation, turn-on and turn-off of electromagnetic relays cannot be accurately controlled and, for practical purposes, are random with respect to the line voltage. Turn-on of a relay coincident with the peak of the line voltage can result in significant inrush currents (I=C*dV/dt) into capacitive loads (i.e., charging of capacitors in connected equipment DC power supplies). This inrush current can result in transients that can affect both connected and nearby equipment. Turn-off of the relay coincident with the peak of the connected equipment load current can also produce transients (V=L*di/dt) that can affect both connected and nearby equipment.
In addition to the transients resulting from random turn-on and turn-off of the electromagnetic relay, transients and noise are also produced as a result of contact bounce. Contact bounce results when the relay coil is energized causing the normally open contacts to come together and bounce off each other several times before finally coming to rest in a closed position. Similar to the contact bounce associated with turn-on of the relay, contact bounce can also occur when the relay is turned off. Associated with turn-on and turn-off contact bounce is arcing between contacts as they come close together or start to separate. The amount of arcing depends on both the supply (i.e., line voltage) and load conditions (i.e., load current). With contact bounce occurring coincident with the peak of the line voltage, contact arcing can be significant, causing pitting of the contacts and a reduction of the effective lifetime of the relay.
The use of electromagnetic relays in conventional automatic switch circuits, therefore, results in the following: random turn-on related transients; random turn-off related transients; contact bounce related noise and transients; and reduced relay lifetime as a result of contact arcing.
With regard to overvoltage circuits, some conventional automatic switch circuits include an overvoltage circuit that controls the relay control circuit to turn off the electromagnetic relay in response to swell and overvoltage conditions. In addition to one or more resistors, capacitors, diodes, and Zener diodes, conventional overvoltage circuits typically rely on one or more discrete solid state switching devices (e.g., bipolar transistors, SCRs, etc.) to control the relay control circuit to turn on and off the electromagnetic relay. As will be explained, the problem with such conventional overvoltage circuits is the variation in performance (cut-out voltage) resulting from ambient and component temperature changes.
FIG. 2 shows a typical overvoltage circuit which is used to illustrate these problems. In this circuit, resistor R101 and R102 are connected in series between the line and neutral conductors, and capacitor C101 and diode D101 are connected in series along a path parallel to resistor R101. The emitter of a bipolar transistor Q101 is coupled to the line conductor and the collector of transistor Q101 is coupled to ground via serially-connected resistor R104 and diode D102 and serves as an output to the relay control circuit. The base of transistor Q101 is coupled to a node between capacitor C101 and diode D101 via serially-connected resistor R103 and Zener diode Z101. The relay control circuit includes SCR Q102, capacitor C102, resistor R105, and diode D103 connected in series between the line and neutral conductors. The output of the overvoltage circuit (from the collector of transistor Q101) serves as the gate drive current for SCR Q102. Capacitor C102 is connected in parallel with the relay coil of a relay K101 with normally open contacts arranged in-line with the line conductor between the supply and load.
Resistors R101 and R102, diode D101, and capacitor C101 are configured as a line-to-neutral connected, unregulated, half-wave rectifier with a DC voltage across capacitor C101 that is directly proportional to the line voltage (line-to-neutral voltage). Under nominal line voltage conditions, the voltage across capacitor C101 is less than the reverse breakdown voltage of Zener diode Z101. As such, there will be no base drive to transistor Q101. With transistor Q101 off, gate drive to SCR Q102 is created through diode D102 and resistor R104. With the SCR Q102 on, the relay control circuit is connected across the line/neutral pair and relay coil voltage is produced across capacitor C102. If sufficient, this relay coil voltage will cause the normally-open relay contacts of relay K101 to close, thereby passing power to connected equipment.
As the line voltage rises (i.e., in the case of a swell or overvoltage condition), so does the voltage across capacitor C101. When the voltage across capacitor C101 exceeds the reverse breakdown voltage of Zener diode Z101, then current through resistor R103 will result (i.e., transistor Q101 base drive current). The amount of transistor Q101 base drive current will depend on the extent of the swell or overvoltage condition. When the line voltage is such that the voltage across capacitor C101 just exceeds the reverse breakdown of Zener diode Z101, then the resulting base drive current will be relatively low. As the line voltage increases, so does the base drive current to transistor Q101. With this circuit configuration, transistor Q101 operates more like an amplifier than an on/off switch. As base drive to transistor Q101 increases, so does the transistor Q101 collector current. As the transistor Q101 collector current increases, the gate drive current to SCR Q102 decreases. As SCR Q102 gate drive current drops, the phase controlled configured SCR Q102 will be on for a shorter proportion of each line voltage half cycle which will cause the relay coil voltage to drop. This type of discrete semiconductor overvoltage circuit, typical of conventional designs, functions by lowering the relay coil voltage as the line voltage increases beyond the point at which the Zener diode Z101 first starts to conduct. At a certain line voltage (cut-out voltage), the relay coil voltage drops to a point where the relay core magnetic force is insufficient to keep the normally-open relay contacts closed, and the relay contacts open.
If circuit values of the various components remained constant, this circuit approach would provide a consistent response to swell and overvoltage conditions. Circuit values are not constant, however, and can vary significantly with changes in component temperature, for example. The relay coil resistance, for instance, can vary by more than 20% as the relay temperature rises from 25° C. to 50° C. Because the coil resistance determines the coil current (and relay core magnetic force) for a given relay coil voltage, changes in coil resistance can dramatically affect the relay coil voltage necessary to maintain the normally-open relay contacts in a closed state. As a result, changes in ambient and component temperature can significantly affect the operation of such discrete semiconductor overvoltage circuits. Testing of these types of circuits shows significant changes in cut-out voltage (10% reduction) as the temperature of the relay rises from room temperature to full-load temperature (i.e., the temperature that the relay rises to after one hour at full resistive load).
The problem with this degree of temperature related cut-out voltage variation is that, if the device in question heats up as a result of continuous full load, the cut-out voltage could drop to a level where nuisance tripping (turning off) of connected equipment could result. For devices that are designed to improve connected equipment uptime, these types of overvoltage circuits represent a significant limitation.